On Sun, Jun 24, 2007 at 11:24:16AM +0100, Hugh Dickins wrote: > On Sun, 24 Jun 2007, Russell King wrote: > > On Fri, Jun 22, 2007 at 07:39:33PM +0100, Hugh Dickins wrote: > > > I'm quite happy with this approach for 2.6.23-rc, along with your ARM > > > dma_map patch which (if I understood aright) rmk approved. > > > > I didn't approve it. Please re-read my reply - there are still some > > unanswered questions in it which _really_ need answering. > > Sorry for misrepresenting you. > > > The report talks about the AT91 machines. These machines do not have > > cache coherent DMA. Therefore, the code being patched should be > > optimised away by the compiler. *Or* we have even bigger problems. > > > > Please forward the original problem report. > > Done.
Okay, that seems to back up my suspicions - it's definitely AT91-based. Since AT91-based machines do not have a DMA coherent cache, arch_is_coherent() must be defined to '0'. The only way that kmalloc could be reached is if that were defined to something other than '0', and if that's done on a machine with DMA incoherent caches, that will lead to data corruption. I think we need to wait for Nicolas to respond on this issue before running headlong into applying a sticky plaster for something which is actually a deeper issue. However, the arch_is_coherent() path _is_ buggy as it stands, but in more than the way identified thus far. Eg, it doesn't set __GFP_DMA appropriately for various DMA masks, so it might return DMA inaccessible memory. If we're after a simple fix for 2.6.22, the _easiest_ solution would be to delete the entire arch_is_coherent() branches in arch/arm/mm/consistent.c; that will result in a working solution for everyone, albiet at a slightly lower performance for the DMA-coherent CPUs. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
